When you’re out hiking in the Grand Canyon, you never know what you might see. You could cross paths with lizards, tarantulas or maybe even something bigger like a javelina. More likely, you’ll also come across the tracks of these critters.

But imagine you’re hiking in the Grand Canyon, and you stumble upon a slab of fallen rock. On it are some odd indentations like overly baked footprints. That’s exactly what happened to a group of hikers on the Bright Angel Trail.

A set of 28 small footprints were discovered on a slab of rock that had fallen from the canyon wall. It turns out that the set of tracks is about 310 million years old – nearly 250 million years before the age of dinosaurs. Steve Rowland, a professor of geology at the University of Nevada, Las Vegas, said the find is remarkable.

“There are some trilobite tracks and other little arthropods and things older. But in terms of actual vertebrate animals these are the oldest by far. Nobody’s ever found any tracks this low, this old in the Grand Canyon,” Rowland said.

Until now, no reptile tracks have ever been found in this type of rock before in the Grand Canyon. Rowland said paleontologists such as himself have never bothered to look in that area for ancient animal tracks since none have ever been found. But this discovery is a game changer.

“Now that we know there were animals walking around … at that time period we can … look at some other places and see if we can find any additional tracks.”

But what animal made those tracks is still a mystery. Rowland said it’s definitely some kind of prehistoric reptilian creature, which was living at the very beginning of reptile evolution.

“It looks like a reptile track. Amphibian tracks tend to have short stubby little toes like salamander toes for example or toad toes. These are long skinny toes that look much more reptilian,” Rowland said. “So I think this is some sort of reptile but it’s extraordinarily early just the reptiles were just appearing at the time that this animal was walking around.”

He said it might have looked like some sort of small reptile, but we won’t really know without finding the animal’s bones. Rowland’s team will publish their research in January.